Abstract: A nonlinear state space model for reactor-regenerator system of FCCU in a large scale refinery has been developed through first principle analysis and system decom-position,The global system model consists of a number of subsystems with both state and control coupling.The resulting model has the merits of structure simplicity and lower dimen-sionarity.The paper also gives the comparison between industrial test data and themodel based computer simulation,which shows satisfactory accuracy for optimization applications.